Structured Safety

Approaches to confined space work

by Shailesh Purohit, IOSH


Confined space (CS) entry has been recognised as a hazardous activity for many years and yet, sadly, it continues to be a source of major accidents and incidents resulting in fatalities and severe environmental pollution around the world.

When you search online, there are a number of examples of fatal CS-related accidents. You may be familiar with such incidents, either as first-hand experience or from colleagues. As examples:

  • Four people died when entering a sewage pit for cleaning as a result of trying to rescue the first entrant who got into difficulties (Gulf News, 2015)
  • Three people were killed in ADCO onshore oilfield H2s accident

What is a confined space?

Although a straight forward question, the definition differs from country to country. But the essential features of a confined space such as enclosed space, risks and differentiation from other parts of the establishment are common to all.

For example, under the Abu Dhabi OSHAD (CoP 27) Code of Practice V3.1 March 2019, and the UK HSE Approved Code of Practice (L 101), a confined space is defined as “any place, including tanks, vessels, pipes, sewers, silos, storage bins, hoppers, vaults, pits, excavations, manholes or other similar space that by the virtues of its enclosed nature, there arises a reasonably foreseeable specified risk”.

Under OSHA 3138-01R Standard, a CS is defined as somewhere that:

  • Is large enough for an employee to enter fully and perform assigned work
  • Is not designed for continuous occupancy by the employee
  • Has a limited or restricted means of entry or exit These spaces may include underground vaults, tanks, storage bins, pits and diked areas, vessels, silos and other similar areas.

Furthermore, the OSHA standard narrows it down to Permit Required type of Confined Space (PRCS) as follows:

  • Contains or has the potential to contain a hazardous atmosphere
  • Contains a material with the potential to engulf someone who enters the space
  • Has an internal configuration that might cause an entrant to be trapped or asphyxiated by inwardly converging walls or by a floor that slopes downward and tapers to a smaller cross section
  • Contains any other recognised serious safety or health hazards

Confined space locations

Some features of a confined space are not obvious at first glance. For example, when entering a space enclosed on three-sides by land but open to the sky it may not seem like a CS.

There are, however, recorded fatalities in such spaces in the farming industries or by the side of canals where the level of oxygen may be below the safe limits due to displacement by biological plant/animal activities, which could release low-lying carbon dioxide or hydrogen sulphide.

The more common locations of a confined space are listed below based on L 101 UK HSE guidance referenced in Section 3. Note that the list is not exhaustive. If your specific CS does not appear on this list, it does not mean

it is not a confined space. For more information, details of how to identify a CS are provided in Section 8 of the HSE guidance.

Examples of confined space locations include:

  • Ducts, culverts, tunnels, boreholes, bored piles, manholes, shafts, excavations and trenches, sumps, inspection and under-machine pits, cofferdams
  • Freight containers, ballast tanks, ships’ engine rooms and cargo holds
  • Buildings, building voids
  • Some enclosed rooms (particularly plant rooms) and compartments within
  • Enclosures for the purpose of asbestos removal
  • Areas used for storage of materials that are likely to oxidise (such as store rooms for steel chains or wood pellet hopper tanks)
  • Unventilated or inadequately ventilated rooms and silos
  • Structures that become confined spaces during fabrication or manufacture
  • Interiors of machines, plant or vehicles

As a reminder, there may be other types of confined space not covered in the previous list.

Hazards of confined spaces entry

The more common hazards of a confined space are listed as:

  • Presence of flammable or oxygen rich environment
  • Presence of toxic and/ or corrosive substances
  • Lack of oxygen or depleted levels of oxygen
  • Presence of asphyxiants
  • Thermal load (hot or cold)
  • Working at height
  • Unexpected ingress of other substances or water
  • Other hazards such as electricity, noise, collapse or subsidence of or within the space, loss of structural integrity and those arising from mechanical equipment and working space
  • Hidden presence of toxic, corrosive or flammable substances below apparently dry films such as previous tank coating, which during cleaning processes become live and emit the trapped substances

Modern Standards and Regulations

Countries around the world have their own safety standards on CS entry requirements. Examples for the Middle East, Europe, Australia and USA, respectively, are listed below:

  • 27.0 – Confined Spaces v3.1 English- Updated available at OshadSystemDocument/Attachments/122/OSHAD-SF%20-%20TG%20-%20 Safe%20Work%20in%20Confined%20Spaces%20v3.1%20English.pdf
  • Safe Work in Confined Spaces (
  • Model Code of Safe Working (Confined Spaces available at: www.safework
  • OSHA (29 CFR 1910.146) Confined Spaces and Permit-Required Confined-Spaces OSHA 3138-01R (2004) (

This is not an exhaustive list and some situations would present additional or multiple simultaneous hazards such as ergonomic hazards whilst carrying out detailed maintenance in confined spaces with power tools.

Identification of a confined space

The first key step is the identification of the confined space. Confined spaces are identified in many different ways around the world but, in essence, the higher the severity of consequence (fatality), the more severe is the classification of the confined space.

With the UK as an example, there are National Occupational Standards for Confined Spaces. These classify the CS as one of the following three classes according to a presentation delivered at the IOSH Joint Conference on Management of Confined Spaces.

Low risk

Low risk shallow entry with adequate natural or mechanical ventilation and where the access is simple and unobstructed and there is no likely risk of flooding. For example, meter pits, stairwells, valve chambers or boosterpumping stations.

Medium risk

There is a medium risk that a specified hazard may be present. This level of risk will require the use of escape breathing apparatus, and will have one or more persons positioned outside the confined space with responsibility for controlling entry and dealing with emergencies.

Entry into medium risk confined spaces may involve more than one person and can be:

  • Vertical entry, normally using a ladder or man-riding winch, requiring fall protection and a means of assisted rescue such as a rescue winch
  • Horizontal entry, such as walking or climbing or working away from the bottom of a vertical entry with limited use of a rescue line

High risk

The following illustrates a high risk confined space entry:

  • Non-standard entries making rescue difficult
  • It is likely that a hazard will be present at some time during the entry
  • May involve complex entry procedures with specific control and rescue arrangements
  • Will entail the use of self-contained breathing apparatus or airline, detection equipment and resuscitation equipment
  • Will require the presence of personnel with responsibility for dealing with emergencies

The above classifications of a confined space provide a useful reminder of the key hazards/ precautions to be followed when devising a safe system of work

Confined space entry process risk assessment

When considering tasks inside a CS, the first question to ask is whether it is strictly necessary to enter it to do the task. For example, could a task such as cleaning be safely carried out from outside of the confined space in certain circumstances?

The UK HSE recommends a hierarchical approach to managing risks, as per Table 1. Personal Protective Equipment (PPE) is generally regarded as a last resort if the measures listed from 1 to 4 do not work. However, there are many cases where PPE is a mandatory additional protective measure and must be used.

To conduct a suitable and sufficient risk assessment, consider the following steps in the order listed:

  1. Identify the confined space and classify it according to one of the three classifications as listed previously.
  2. Consider the risk control hierarchy shown in Table 1.
  3. If entry into the confined space is unavoidable after the above considerations, then carry out the risk assessment based on the following factors as per Health and Safety Executive, UK 2014, Safe Work in Confined Space. This is available from

Considerations include:

a. General condition of the confined space: i. Previous contents (are these toxic, flammable, corrosive?) ii. Residues (will these react with any cleaning materials?) iii. Contamination (will this emit harmful vapours or reduce oxygen?) iv. Oxygen deficiency and oxygen enrichment (will this increase risk of fire or asphyxiation?) v. Physical dimensions (can anyone entering the space work safely and ergonomically?) b. Hazards arising from the work i. Cleaning chemicals (health/ flammability/ corrosive effects) ii. Sources of ignition (static precautions/ non-sparking tools/ electrical heating/lighting) iii. Increasing temperature (hot work such as welding/ cutting/ chemical reactions) c. Hazards from outside the space i. Ingress of substances (solids/ liquids or gases) d. Emergency rescue (anticipate and provide for foreseeable emergencies)

Whilst the use of checklists based on above factors is a reasonable way forward, beware of relying entirely on such checklists, as the risk assessments should be site specific and may have a dynamic situation where risks not included in the standard checklist may be present.

Once it is concluded that entry to confined space is not avoidable and a risk assessment, at the exact location where work is to be carried out, has been conducted, it is time to consider a safe system of work, also known as “Permit to Work”. These are required for non-routine, non-production tasks involving transfer of responsibilities between two or more groups.


One of the excellent references for designing and operating a Permit System is available as HSG 250 from the Health and Safety Executive, UK 2005, Guidance on Permit-to-Work Systems. This is available from www.hse.

Permits should not be viewed as a paper-based administrative task to be carried out in one’s office, but should be treated as an important communication tool for exchange of key hazards/ risks and precautions between all stakeholders who may be affected by the tasks.

Note that there may be multiple permits issued for the same task. For example, welding a broken mechanical support inside a large tank will require Hot Work, General Entry, and Work at Height permits. Figure 1 summarises different type of permits.

HSG 250, as referenced above, provides examples of various check lists and permit documents and also highlights the importance of having a structured process to issue, maintain and close out permits in Appendices 2, 3 and 4.

Roles and Responsibilities of Permit Issuer, Permit Acceptors and Permit Auditors must be defined in the Permit Policy/ Procedures. Information, Instruction, Supervision and Training (IITS) are key to designing and maintaining an effective Permit to Work System.

Closing out a permit is a safety critical step and must always be completed when the CS work has been concluded.

The failure to do so has resulted in many a fatal incident, for example the Piper Alpha oil rig explosion in 1988.

PPE and RPE guidance

As part of pre-entry into confined spaces, the risk assessment must consider the most likely hazards in terms of presence of chemicals (toxins, flammables and toxics), working temperature (heat/ cold stress), electrostatic ignition, ignition sources, sharp objects, and fumes generated as part of work processes such as welding.

For respiratory hazards, selection, donning and maintenance of respiratory protective equipment (RPE) is a safety critical requirement which must be assessed and authorised by a competent person. A competent person for these purposes will be someone with the necessary skills, knowledge and experience of, and familiarity with, the relevant processes, plant and equipment. In complex cases more than one person may be needed to assess the risks relating to specific areas.

RPE guidance is provided in detail in HSG 53 from the Health and Safety Executive, UK 2013, Respiratory protective equipment at work: A practical guide. This is available from: www.hse. A summary of all the usual types of RPE available and used in the UK is provided in Figure 2.

The types of filters commonly used in the UK (having equivalent types in other parts of the World) are listed below taken from HSG 53 referenced previously in Figure 2.

You must ensure that the chosen RPE meets national or international standards to provide full protection to users.

RPE “Fit Testing” for certain categories of respirators is mandated in the UK. This ensures that the respirator “fits” the wearer adequately so as to provide full protection.

There are compulsory qualitative and/ or quantitative fit tests. More information is available in the HSG 53 Guidance referenced in this article. It is not acceptable or legal to just issue a mask to a worker and ask them to get on with the task.

Gas testing and PPE requirements

Before entering a CS, which is suspected of containing harmful atmosphere, the entry permit must mandate gas testing to enable the specification of correct RPE. The gas monitoring results must be recorded periodically until the task is completed on the Permit Entry document.

A worked example is provided in Appendix 5 of HSG 53 Guidance on how to select a suitable RPE.

Readers may find RAE Systems 2014, GUIDE TO ATMOSPHERIC TESTING IN CONFINED SPACES [Online]. This is available from https://www.raesystems. com/sites/default/files/content/ resources/Application-Note-206_GuideTo-Atmospheric-Testing-In-ConfinedSpaces_04-06.pdf uk/pubns/books/hsg53.htm.

This is an excellent tool on how to carry out gas monitoring for various common hazards. It highlights critical oxygen levels, carbon monoxide and hydrogen sulphide, as well as flammable atmospheres and provides background information on which gas monitors to use.

Emergency rescue and standby person(s)

When carrying out the confined space entry risk assessment, emergency rescue arrangements must be considered and put into place based on the foreseeable emergency situations. For example, a standby person must be available as a fire watcher when a confined space is assessed to contain flammable atmosphere. The rescue person must also be fully trained in safe evacuation of the people inside the danger area.

Multiple fatalities have occurred when rescuers have also been killed trying to rescue those in difficulties.

Author Details

Shailesh Purohit, IOSH

Shailesh Purohit is a Chartered Fellow of the Institution of Occupational Safety and Health (IOSH), with over 35 years of experience in the Process Industries. He has a keen interest in Process Safety and represents IOSH’s Hazardous Industries Group on the UK National Process Safety Forum. Bow Tie Assessment is of particular interest and passion. He is currently working as a Process.